The invention relates, generally, to personal communication services (PCS) systems and, more particularly, to a method and apparatus for providing interswitch handover, or automatic link transfer, in such systems.
As will be appreciated, PCS systems provide wireless communication services. PCS systems, unlike other wireless services such as cellular telephone networks, are intended to provide wireless service for low mobility applications where the users of the PCS move relatively slowly over limited geographic areas. For example, PCS systems are intended to provide wireless service within business complexes, large buildings, commercial parks, city business districts, universities or the like where the customer will be walking or otherwise moving slowly in a limited area. Because PCS is specifically designed for low mobility applications, it does not require the more extensive and expensive infrastructure of wireless systems such as cellular telephone networks. As a result, PCS wireless service can be provided at lower cost.
The typical PCS system, defined in Bellcore Generic Requirements GR-2801-CORE Issue 1, Dec. 1993, consists of a number of radio port control units (RPCU) each controlling a radio port having a radio transmitter/receiver for communicating with mobile stations located in the geographic service area served by that radio port. Multiple radio port control units, arranged such that their service areas are contiguous, are provided to extend the range of service beyond the geographic area served by a single RPCU. The RPCUs are connected to and communicate with switching systems over lines that transmit voice and call control signals therebetween. The switching systems are connected to, and form part of, the public switched telephone network (PSTN). Thus, PCS users are provided with wireless access to the public switched telephone network via a relatively inexpensive system.
It is to be understood that RPCUs serving contiguous geographic areas can be connected either to the same switching system or to different switching systems. When a mobile station moves between areas served by RPCUs on the same switching system, the transfer of service from the current RPCU to a new RPCU is known as an intraswitch handover or an intraswitch automatic link transfer (ALT). Such intraswitch ALTs are relatively simple to control by setting up a three-way connection in the switching system between the line or trunk from a remote station on the PSTN, a line to the current RPCU and a line to the new RPCU to maintain connectivity between the mobile station and the remote station while effectuating the handover. Once the new RPCU is serving the call, the three-way connection is broken by disconnecting the line to the first RPCU. Because a single switching system hosts both the current RPCU and the new RPCU, it recognizes the directory numbers of the lines handling the call from both RPCUs and is aware of the lines'physical location in the switching system such that the switching system can easily set up the three-way connection.
When a mobile station moves between geographic areas served by RPCUs hosted by different switching systems, the transfer of service from the current RPCU to the new RPCU is known as an interswitch handover or an interswitch ALT. Interswitch handovers have proven to be much more difficult to control than intraswitch handovers because the switching system hosting the new RPCU must complete the handover based on information that identifies the call in other switching systems. The problem of providing interswitch handover is exacerbated by the fact that standard operating systems and protocols are already in place for much of the PCS system such that any method for handling interswitch handovers must not disrupt or alter existing protocols and/or operating systems. For example, existing protocols require that an active mobile station be provided with the directory number identifying the location on the switching system of the line currently serving the call but not with any identification of any prior switching systems. Any method for handling interswitch handovers must not alter this or other similar protocols and/or operating systems. Thus, the switching system hosting the new RPCU, based on the directory number provided by the mobile station, must return to the switching system hosting the current RPCU to locate the call and set up the three-way connection which must be created to maintain connectivity between the remote station and the mobile station during the handover. Where a mobile station travels to RPCUs on more than one new switching system, connections between each of the prior switching systems must be maintained. Moreover, it is possible that the call will reappear at a switching system that hosts a prior connection, a process known as shoelacing, where a single switching system maintains two or more connections for a single call. It will be appreciated that the need to maintain all prior connections results in the allocation of a significant amount of valuable network resources for each call and unnecessarily complex routing. Finally, it is extremely difficult, and requires complex protocols, to provide supplementary services such as conference calling and call waiting where a plurality of switching systems are connected together. Further, while PCS and cellular systems both provide wireless communications service, the standards and protocols governing the operation of PCS differ from those of cellular such that existing cellular techniques are not transferable to PCS.
Thus, a problem in the art exists in that a satisfactory method for handling interswitch handovers in a PCS system that is compatible with existing PCS protocols has not been developed.